Electric arc welding is a complicated process and the resulting deposition of molten metal into a weld pool for performing the welding operation is determined by a tremendous number of interrelated and non-interrelated parameters. These parameters affect the deposition rate, the spatter and debris around the welding operation, the shape and appearance of the weld bead, and the location and quality of the protective slag, to name just a few. The welding process is controlled by the protective gas composition, its flow rate, torch design, the welding torch angle, welding tip design, the size and shape of the deposition groove, control apparatus used in the welding process, amount of stick-out, wire feed speed, speed of the torch along the workpiece, smoke extraction, type of grounding contact on the workpiece, atmospheric conditions, the composition of the workpiece and other variables. Consequently, arc welding has been largely a trial and error procedure with the ability of the welder to use the appropriate settings for obtaining consistent welds. Each time one of the parameters is changed, the appearance, size, shape, contour, chemistry and mechanical properties of the resulting weld is affected. For this reason, arc welding is not a precise science, but rather an art form requiring trained welding engineers to provide the desired results. Most systems employ electrical welding parameters at the welder itself, such as a closed loop control based upon arc voltage, arc current or pulse settings. The settings of voltage, current or pulse size or rate are controlled by the welding engineer or by the technician for generating the desired welding. There is no procedure in the art which controls a D.C. welding process ad hoc without the intervention of the welder or welding engineer. Consequently, in high production D.C. welding the weld is controlled by adjusting various primary parameters and disregarding the less meaningful parameters.
In summary, automatic arc welding using a D.C. arc welding process is normally controlled by the welder in a manner that will not accomplish uniform welding results with variations in one or all of the many welding parameters or variables.
In a D.C. arc welding process of a voltage control mode, to which the present invention is particularly directed, it is known that semi-automatic and automatic welding can be controlled at a constant arc voltage. If this voltage is relatively high as shown by the voltage trace in FIG. 1, a relatively constant arc voltage can be maintained with very little deviation from the norm and without apparent or significant voltage deflections. When operating at this high voltage, the metal transferred in the plasma arc welding process is by spray transfer wherein very small, liquid metal streams from the wire to the workpiece in the arc itself. The weld puddle is observed to be relatively quiet. This makes high voltage D.C. arc welding at a constant voltage quite inviting. However, the high voltage of the plasma arc welding as shown in FIG. 1 generates excessive heat and electromagnetic radiation. Additionally, it evaporates the iron of the workpiece as well as the iron of the electrode or advancing wire. This iron vapor is oxidized in the high temperature of the arc to an iron oxide aerosol which condenses on the relatively cool workpiece surface primarily as iron oxide dust. This produces a somewhat "dirty" welding process and is not considered to be optimum for the D.C. welding procedure. If the constant voltage setting for the arc welder is reduced substantially from the set voltage in FIG. 1 to the constant voltage illustrated in the voltage trace of FIG. 2, a globular or short circuit D.C. welding process is performed. This constant voltage process is characterized by a large number (over about 80 per second) of significant negative voltage deviations. Indeed, when short circuits occur, the arc voltage plunges to a voltage near zero, i.e. 7-10 volts, and may stay there for a long time, i.e. until the short is broken. As a result the wire advancing toward the workpiece is driven into the weld puddle to form a short circuit. When this happens, the arc is extinguished and the wire heats up and explodes. The explosion breaks electric contact and the voltage immediately shifts to the high set voltage, which is normally overshot due to the inductance of the welding power supply. When the short is broken and the voltage shifts back toward the set arc voltage, the wire is still rapidly moving toward the workpiece. Thus, the arc is reestablished and this cycle is repeated. Consequently, great voltage instability occurs immediately after the short and the break referred to as a "neck" before the electric arc or plasma is again established. This mode of metal transfer is primarily "globular" in nature with a great number of negative dips, but is also a "short circuit mode" with over 100 shorts per second. These processes create chaotic action in the molten metal forming the weld puddle. When actually in the short circuit mode of operation, the wire or electrode shifts up and down so that the molten metal on the end of the advancing wire causes a short circuit. Thus, the arc voltage shifts to zero and extinguishes the plasma. Repeated short circuit and/or globular transfer operations of the welder causes drastic variations in the arc voltage of the D.C. arc welding process. The liquid globules are blown away from the arc due to the turbulence caused by the plasma and are deposited onto the workpiece where they solidify. Consequently, large hemispheres called "spatter" are adjacent the weld bead and must be chiseled off to make the weld area more attractive. The use of a constant voltage which is high for spray transfer, as shown in FIG. 1, or is low for globular or short circuit transfer, as shown in FIG. 2, presents unwanted welding results so welding is to be performed in the specific area between spray transfer and globular transfer; however, the many variables in the welding process change the voltage defining this area for a weld process. If a voltage is set for one group of variables, the weld process is not optimum when one or several of these variables change.
Although the present invention is particularly applicable to voltage control welding, the same problem exists for current control welding. In pulse welding, the many variable parameters also affect the quality of the weld and requires both quality control of the wire and shielding gas as well as adjustment of the other variable parameters. These compensations in all types of arc welding cannot be made using a closed loop, adaptive system based upon a given variable or an open loop control system such as constant voltage, constant current or constant pulse wave shape.